The storage capacity of longitudinal magnetic recording media has increased considerably due to the reduction of media noise and the development of high-sensitivity spin-valve heads and high-magnetization write heads. Recording densities above 50 Gbits/inch2 have been demonstrated, and such high recording densities are on the verge of being applied for commercial hard disk drives. The demand for greater recording densities for better performing computers is however showing an increasing trend imposing greater challenges for the recording media and other component design.
Lowering the media noise involves writing sharper magnetic transitions in the magnetic layer. This is generally achieved by increasing the media coercivity, decreasing the thickness of the magnetic layer, decreasing the grain size and grain size distribution of the magnetic layer, and magnetically isolating the grains of the magnetic layer.
However, decreasing the grain size or decreasing the media thickness adversely affects the thermal stability of the magnetic recording medium. The thermal stability of the magnetic layer is normally represented by how large the factor KuV/kT is, where Ku denotes the magnetic anisotropy, V denotes the volume of the grain, T denotes the temperature, and k denotes the Boltzmann constant. In order to obtain small grains which are thermally stable, the magnetic anisotropy Ku has to be increased.
The magnetic anisotropy field Hk is defined by Hk=2Ku/Ms, where Ms denotes the saturation magnetization. A large magnetic anisotropy field Hk means a large coercivity Hc at the nonosecond regime where the writing of the information normally occurs for high recording density magnetic recording media with high data transfer rates. But a high coercivity Hc at the writing frequencies puts severe limitations on the write head, as a large write current is required in order to write the information on such magnetic recording media. The write current of the write head is severely limited due to difficulties in developing high magnetic moment write heads. The overwrite performance, which is the ability to write new data on previously written data, is worse for the magnetic recording media with a high magnetic anisotropy field Hk. Further, the magnetic recording media with a high magnetic anisotropy Ku increases the magnetic anisotropy field Hk, thereby restricting the overwrite performance.
As described above, there is a need to decrease the grain size of the magnetic layer and the thickness of the magnetic layer in order to achieve the low media noise and the high density recording performance. However, decreasing the grain size and the magnetic layer thickness deteriorates the thermal stability of the magnetic recording medium.